1. Field of the Invention
This invention relates to radiant energy collection apparatus and, in particular, to optical concentrators for solar radiation.
2. Description of the Prior Art
Spheroid or paraboloid reflectors have often been used for concentrating solar radiation to a small or a point focus in solar collectors. However, a large area reflector is required to obtain high heat flux at the focus. Whether formed in one piece or in sections, large area reflectors are generally difficult to make with the requisite accuracy, and are expensive.
Echelon reflectors are an alternative to the spheroid or paraboloid approach. Typically, the echelon reflector uses circular grooves to focus solar rays. Such an annular echelon reflector has the advantage of being a relatively thin, flat panel which can be mass produced accurately. Unfortunately, due to the limitations of the ruling lathes which produce the master for these reflectors, reflector size is limited. Hence, the available power density is also limited.
Linear echelon reflectors or linear echelon refractors have also been used as solar radiation concentrators. This type of reflector or refractor concentrates incident solar radiation to a line focus and is thus useful for tubular absorbers. See, e.g., U.S. Pat. No. 3,915,148 issued Oct. 28, 1975, to Fletcher, which teaches the use of cylindrical Fresnel lenses to focus solar radiation on linear collector or absorber elements. Also, lens and mirror elements have been used cooperatively to focus radiation on a linear collector, as taught in U.S. Pat. No. 3,125,091 issued Mar. 17, 1964, to Sleeper. However, the flux density at a line focus is less than that for a comparable small area or "point" focus and limits the application of such line focus collectors.
As will be appreciated from the foregoing discussion, it is highly desirable to have a solar energy concentrator which combines high flux density with ease of manufacture, even for large area concentrators.